Weak Values, Quantum Trajectories, and the Stony-Brook Cavity QED experiment

Weak values as introduced by Aharonov, Albert and Vaidman (AAV) are ensemble average values for the results of weak measurements. They are interesting when the ensemble is preselected on a particular initial state and postselected on a particular final measurement result. I show that weak values arise naturally in quantum optics, as weak measurements occur whenever an open system is monitored (as by a photodetector). I use quantum trajectory theory to derive a generalization of AAV's formula to include (a) mixed initial conditions, (b) nonunitary evolution, (c) a generalized (non-projective) final measurement, and (d) a non-back-action-evading weak measurement. I apply this theory to the recent Stony-Brook cavity QED experiment demonstrating wave-particle duality [G.T. Foster, L.A. Orozco, H.M. Castro-Beltran, and H.J. Carmichael, Phys. Rev. Lett. {85}, 3149 (2000)]. I show that the ``fractional'' correlation function measured in that experiment can be recast as a weak value in a form as simple as that introduced by AAV.